The present application relates to the electronic lighting arts. More specifically, it relates to lamp ballast circuits and, in particular, to high intensity discharge (HID) lamp electronic ballasts. One particular application is to use such a ballast in an automobile headlamp assembly, and the present application will be directed with particular attention thereto.
HID lamps are considered to be one of the most effective light sources. These lamps have high electrical to lumen efficiency, long life, good color rendition and good focusing capability when the arc is made short. These favorable characteristics, and in particular the very high brightness and color temperature of commercial HID lamps, make them good candidates for sophisticated applications such as automotive headlights. Application of HID lamps in such demanding environments, however, is far form straightforward due to the many peculiarities of HID light sources. An issue with HID lamps is the need for special ballasts to drive them.
Many lamps have a relatively narrow band of power in which they can operate, and require ballast circuits to rectify, filter, and convert power from a source. Thus, ballast circuits require heat generating components such as transistors, transformers, and the like. The more complex a ballast circuit is, generally the more heat it will produce, and the more likely it is for one of the components to fail. Additionally, the more complex a ballast is, generally the more it will cost. Ballast designers struggle to find the simplest designs to produce a ballast that supports particular lighting applications. Less complexity, and fewer parts lead to a less expensive, more robust and commercially viable ballast circuit.
The reliability of a ballast circuit is of increased importance in the particular application of automobile headlamps, for obvious reasons. It would be undesirable to have frequent drop outs when a motorist is relying on their headlamps to drive at night. Also, with space being an issue, it is desirable to make the ballast circuit as compact as possible, and fewer components help achieve that goal.
Another drawback of typical ballasts is that they use a single voltage source. Since lamp applications that require a ballast are driven by an alternating current (AC) signal, these ballasts utilize extra circuit components to construct a full bridge inverter for providing the power to drive the lamp. These additional components that make up the bridge inverter add to the problems noted above, such as size, cost, heat, and complexity.
The content described in the present application contemplates a new and improved method and apparatus for a ballast circuit that overcomes the above referenced problems and others.